Yongsug Chung

Reactive wetting phenomena of MgO–C refractories in contact with CaO–SiO2 slag

Abstract Wetting phenomena between MgO–C and CaO–SiO 2 slags were investigated by varying carbon content. A sessile drop technique was adopted to study the wetting phenomena in conjunction with a high speed camera for the observation of intrinsic wetting phenomena. The results show that the high content of SiO 2 and the presence of Al 2 O 3 in slags enhance the diffusion of Mg 2+ , leading to the promotion of reactive wetting. The carbon in MgO–C refractory impedes the penetration of slags by repelling the slag and slowing the diffusion of Mg 2+ . This accounts for the non-wetting behavior of the slag on MgO–C refractory with 17% (mass fraction) carbon similar to that of graphite.

Effect of Rice Husk Ash Insulation Powder on the Reoxidation Behavior of Molten Steel in Continuous Casting Tundish

Rice husk ash (RHA) has been widely used as an insulation powder in steel casting tundish. Its effect on the reoxidation of molten steel in tundish as well as on the corrosion of magnesia refractory was investigated. The reoxidation of the steel, indicated by an oxygen pickup, was progressed by increasing the ratio of RHA to the sum of RHA and carryover ladle slag (R ratio) greater than about 0.2. The increase of the silica activity in the slag layer promoted the self-dissociation of SiO2 from the slag layer into the molten steel, resulting in the silicon and oxygen pickup as the R ratio increased. The total number of reoxidation inclusions dramatically increased and the relative fraction of Al2O3-rich inclusions increased by increasing the R ratio. Hence, the reoxidation of molten steel in tundish might become more serious due to the formation of alumina-rich inclusions as the casting sequence increases. MgO in the refractory directly dissolved into the molten slag layer without forming any intermediate compound layer (e.g., spinel), which is a completely different situation from the general slag-refractory interfacial reaction. A flow was possibly induced by the bursting of gas bubbles at the ash-slag (-refractory) interface, since the silica in the RHA powder continuously dissolved into the molten slag pool. Thus, the RHA insulation powder has a negative effect on the corrosion of MgO refractory.

Effect of Slag Chemistry on the Desulfurization Kinetics in Secondary Refining Processes

Desulfurization behavior was investigated based on a wide slag composition and working temperature range. Moreover, the rate-controlling step (RCS) for desulfurization with regard to the ladle-refining conditions and the transition of the RCS by changing the slag composition was systematically discussed. The desulfurization ratio reached an equilibrium value within approximately 15 minutes irrespective of the CaO/Al2O3 (=C/A = 1.3 to 1.9) and CaO/SiO2 (=C/S = 3.8 to 6.3) ratios. However, the desulfurization behavior of less basic slags (C/A = 1.1 or C/S = 1.9) exhibited a relatively sluggish [S]-decreasing rate as a function of time. The equilibrium S partition ratio increased with an increase in slag basicity (C/A and C/S ratio), not only due to an increase in sulfide capacity but also due to a decrease in oxygen activity in the molten steel. There was a good correlation between the calculated and measured S partition ratios at various slag compositions. However, the measured S partition ratio increased by adding 5 pct CaF2, followed by a constant value. Multiphase slag exhibited a relatively slow desulfurization rate compared to that of fully liquid slag, possibly due to a decrease in the effective liquid slag volume, interfacial reaction area, and a relatively slow slag initial melting rate due to a high melting point. The activation energy of the desulfurization process was estimated to be 58.7 kJ/mol, from which it was proposed that the desulfurization reaction of molten steel via CaO-Al2O3-SiO2-MgO-CaF2 ladle slag was generally controlled by the mass transfer of sulfur in the metal phase. However, there was a transitional period associated with the rate-controlling mechanism due to a change in the physicochemical properties of the slag. For slag with a viscosity greater than about 1.1 dPa·s and an equilibrium S partition ratio lower than about 400, the overall mass-transfer coefficient was affected by the slag properties. Hence, it was theoretically and experimentally confirmed that the RCS of the desulfurization process under secondary refining conditions was strongly dependent on thermodynamic driving forces as well as the viscosity of the slag.

Erratum to: Effect of Slag Chemistry on the Desulfurization Kinetics in Secondary Refining Processes

JIN GYU KANG, JAE HONG SHIN, and JOO HYUN PARK are with the Department of Materials Engineering, Hanyang University, Ansan 15588, Korea. Contact email: basicity@hanyang.ac.kr YONGSUG CHUNG is with the Department of Advanced Materials Engineering, Korea Polytechnic University, Siheung 429-793, Korea. The online version of the original article can be found under doi: 10.1007/s11663-017-0948-2. Article published online April 27, 2017.

Initial Wetting and Spreading Phenomena of Slags on Refractory Ceramics

Wetting angle and spreading rate between slags and refractory ceramics such as Al2O3, MgO, MgO-C or SiC have been recently determined by using dispensed drop technic with a high speed camera. Intrinsic value of wetting angle and the effect of reactions on wetting and spreading are reviewed and discussed. Role of carbide or graphite in refractories are also reviewed when these were in contact with reducible slags. Driving force for spreading and a spreading model (non-reactive viscous model) are discussed.